Offense and defense
The complex of biochemical, physiological, energetic and mechanical systems that make up an organism are extremely vulnerable to threats from the environment, and the greatest danger comes from other organisms. Since the beginning of evolution, each lifeform has been a vital resource for other ones, and it has has te need to defend itself , and to fight back. Camouflage Also see: List of camouflage methods The most secure way to survive predation is avoiding to be recognised as a prey. Camouflage involves an organism modifying its own appearance for this purpose (it's called mimicry when it occurs in such a wayto make it similar to another organism). Crypsis In crypsis, an organism blends itself in the environment, effectively becoming invisible (of course, it's also useful to predators to approach the prey). A disruptive pattern is among the simplest way to do this: the spot on a leopard's fur, or the mottled feathers of a nightjar break up the outline; zebras' stripes meld their shape with the other individuals in the herd. Generally, animals that live in the tall grass have vertical stripes, while those that live among bushes or in the underbrush have broad spots. Often, these animals have dark stripes or marks that hide the eyes, like giant pandas, raccoons, killer whales or common frogs. The shadow is a primary mark of a three-dimensional shape: for this reason, many animals have a darker back, or generally an upper side darker than the lower side; several squids and fish resort to counter-illumination, that is, a ventral source of light. For low and squat organisms, lateral flanges that make the body wide and flat on the ground help to hide the shadow, especially if they have lighter fringed sides. Some animals use self-decoration to hide themselves: the decorator crab covers itself with pebbles, sponges and seaweed, while caddisfly larvae use sand, twigs, or broken shells. Octopi, squids and some flatfish and lizard can actively change colour; foxes, hares and ermines change their coat with the season. A very pecular form of crypsis is found in transparent species, such as comb jellies, young glass frogs, glass knifefish and others (see here). Jellyfish's main tissue, mesoglea, is devoid of cells and rich in water, and thus often transparent, but muscle and other tissues can be made so if the proteins are organized in fibrils smaller than the wavelength of visible light (the effect works best under water). The ice fish has lacks haemoglobin in its blood, making it completely clear, though this requires a greater heart strain and forces it to live in oxygen-rich water. Anyway, some functions (protection from harmful radiations, photosynthesis, sight) require light absorption and their organs can never be invisible. Some fish, such as herrings and marine hatchetfish, can imitate transparency with a flat and thin body covered in silvery, highly reflective scales. Aposematism Aposematism is the opposite of crypsis: rather than hiding, the organisms makes itself stand out in the environment signalling itself as an unappetizing prey. It's most common in toxic species (frogs and salamanders, bees and wasps, beetles, caterpillars, snakes and [http://en.wikipedia.org/wiki/Helodermatidae Heloderma lizards]); it's usually a combination of black and a vivid colour such as red or yellow (or, rarely, blue). Green and brown are avoided, as they're common background colours, but of cours this depends from the dominant flora and geology. In the (venomous) [http://en.wikipedia.org/wiki/Blue-ringed_octopus Hapalochlaena octopi], blue rings appear immediately before the bite. In actually lethal species, though, aposematism is rare, since its success depend from the learning (and therefore, survival) of the predator. Mimesis Mimesis, or true mimicry, involves an organism trying to be as similar as possible to anther one. Examples of cryptic mimicry include stick insects and leaf insects, the leafy seadragon and the mata-mata turtle, or the orchid mantis. Other species that imitate visible but unattractive objects are the spider Celaenia excavata, that mimics bird droppings and several butterflies that look like dry leaves. Batesian mimicry is a form of mimicry in which a non-dangerous organism mimics a dangerous organism. Several snakes, beetles, butterflies, etc. have a false aposematic coloration, for example the moth Sesia apiformis, that has the yellow-black stripes and the transparent wings of a wasp; the hawk-cuckoo imitates true hawks in shape and plumage; several caterpillars and butterflies have eye-like spots that make them look much bigger than they really are. By changing its colour and body shape, the mimic octopus can imitate several dangerous animals, such as sea snakes and lionfish. In mullerian mimicry, several dangerous species adopt the same aposematic coloration, strengthening each other's defense. For example, most bees and wasps have the same yellow and black pattern; the heliconians butterflies have a similar black, yellow and red colour scheme. In the somewhat unusual martensian (or emsleyan) mimicry, a lethal species (say, the coral snake) imitates the pattern of a less dangerous one: the lethal species does not allow the predator to survive, and therefore to learn to avoid it, but the other one does, and makes an aposematic warning actually useful, so that both species can survive without being attacked. Surviving the attack If the prey is sighted and attacked by the predator, it can flee, fly off, hide in a den, or find safety in number: adult musk oxen arrange themselves in a circle with their horns pointing outwards, and the calves at the centre; hamadryas baboons assume a similar arrangement; some gazelles and deer resort to stotting, a particular leap with stretched legs and arched backs that is believed to be a signal of the individual's high fitness; mayflies and anchovies simply live in groups so large that no predator can ever kill more than a very small fraction of them. Even if the attack is successful, the prey can try to avoid consumption. Many animals, such as opossums, pretend to be dead and decomposing, swelling their abdomen and secreting a scent of rotting meat; horned lizards make their eye capillaries burst, squirting blood on the predators (autohaemorrhaging). Others resort to autotomy (self-amputation), often of parts that regrow: lizards lose their tail, crabs their legs, octopi their tentacles, sea cucumbers vomit their own gastrointestinal tract; several species of ants and termites bring this phenomenon to the extreme with autothysis, where organs or venom glands inside some individuals burst, killing them and spilling noxious chemicals outside. Other behaviours focus on distracting the predator: many birds pretend to be injured to lure the predator away from their nest, while plovers incubate a fake nest distant from the real one; blackbirds, corvids and gulls (and some mammals such as meerkats) attack or harass a predator in large groups (mobbing), even though none of them can harm it alone, at least to prevent it to hunt effectively. Most cephalopods can expel ink, a dark melanine-rich liquid, opaque and probably irritating, that hides their escape; in some cases, if the ink is particularly rich in mucus, it can form pseudomorphs, semi-solid shapes that can resemble the animal. Passive defense Physical defense Also see: Exoskeletons Many organism prevent predator from consuming them through an armour that protects them from harm. Often, rigid structural elements are employed (wood in arboreal plants, the exoskeleton on arthropods); a protective external shell can be developed by organisms that otherwise lack rigid support (the lorica of some protozoans, the frustule of diatoms, the shell of mollusks) or that have an internal skeleton (the armour of ostracoderms, placoderms, ankylosaurs and pangolins, the carapace of turtles and armadillos, the scutes of crocodiles). Several plants, such as the holly, have a waxy cuticle on their leaves that makes them harder to chew and digest, and injuries at the stem release resin, a thick hydrocarbon-rich fluid that traps insects and seals open wounds. Another option to mitigate damage is ablative armor, that is, a structure which will come off leaving the rest unharmed. This can range from an actual coating to an approach like shedding a tail in order to escape a predator (see "autotomy" above). Even relatively soft armours can be made more effective with spines that can injure the attacker, as with sea urchins, Murex snails, cacti, horned lizards, hedgehogs, porcupines, etc. Trichomes (surface hollow cells, similar to hairs) are quite common in plants and protozoans; they take the shape of harpoons, arrowheads, hooks or antlers that block or even injure smaller herbivores; often, they release toxic chemicals (such as in nettles; see below). Note that, in botany, a distinction is made between thorns (part of the stem or branches), spines (part of leaves) and prickles (hair-like protrusions). Examples of mutualistic defense are known: the hollow spikes of Acacia collinsii host ants that attack any herbivore; clownfish inhabit sea anemones whose venom does not harm them thanks to a layer of mucus; some species of crabs bring sea anemones on their back. Chemical defense A simple method to be unattractive to predators is concentrating toxins (harmful substances of organic origin) in the tissues. Poison is mainly employed as a defense in plants (toxic animals tend to use venom to kill preys or to strike back at the attacker; see below). See here and here a list of poisonous plants and fungi. Compare the lists of poisonous and venomous animals. While animal toxins are usually a mixture of different chemicals (mostly proteins), poisonous plants use simpler substances that can easily be classified according to their chemical nature. For example: *''Alkaloids'' are strongly basic chemicals derived from aminoacids, and they can activate or inhibit enzymes, interfer with fat and sugar storage or with nervous transmission, bind to nucleic acids to block protein synthesis or outright destroy cells. *''Cyanogenic gycosides, such as the amygdalin found in bitter almonds, are stored in vacuoles and then released together with enzymes that turn them in highly toxic hydrogen cyanide. *[http://en.wikipedia.org/wiki/Terpenoid ''Terpenoids or isoprenoids] are lypids made up by isoprene units; monoterpenoids (2 isoprene units) form essential oils such as menthol, capsaicin, anise and camphor, while diterpenoids (4 isoprene units) are toxic, and often founds in saps and resins. *Phenols are hydrocarbons similar to alcohols that include lignin, cannabinoids and some neurotransmitters, and especially tannins, bitter and astringent chemicals that can interfere with protein digestion. *Other defensive toxins employed by plants include formic acid (found in nettle hairs) and genistein, a substance found in clover that makes sheep temporarily sterile. Attack and active defense Physical attack Chemical attack Toxins used by animals as offensive weapons can be classified according to their effects, such as the kind of biological function they target: *'Hemotoxins' destroy red blood cells (hemolysis), interfere with blood clotting and damage tissues. While the death from hemotoxins is not especially fast, so a predator might have to track the prey after the attack (and of cours eit wouldn't be very effective as a defense) it helps it to digest the prey's tissues by breaking down proteins. Viperid snakes such as rattlesnakes commonly use hemotoxins. *'Cytotoxins' and necrotoxins kill cells, causing the death and decomposition of all living tissues affected by the toxin. The effects are slow, but devastating. The puff adder and the brown recluse spider are examples of predators that employ necrotoxins. *'Neurotoxins' block synapses and sometimes destroy neurons; they can induce death in a fraction of a second. Simple chemicals such as lead, nitric oxide and ethanol can have limited neurotoxic effects; botulinum and tetanus are insted extremely powerful (in fact, they're the two most powerful toxins in the world). Widow spiders, most scorpions, cobras, box jellyfish and cone snails are predators that employ neurotoxins. References